Silver halide crystals have been the dominant photosensitive material in photographic processes for more than a century. During this time, technological improvements in sensitivity have produced a broad range of materials with specialized photographic properties applicable to a broad spectrum of uses.
Modern photographic emulsions consist of a very large number of tiny silver halide crystals dispersed in a polymeric matrix, typically a colloid such as gelatin. Emulsions can be prepared with silver chloride, silver bromide, or silver iodide, or with mixtures of these halides. When light of the appropriate wavelength strikes a silver halide crystal, a series of reactions begins which generates an electron and eventually leaves in the crystal a small amount of free, zero-valent silver. The presence of this free silver in the exposed crystals provides a latent image, which is an invisible precursor of the visible image that is obtained upon subsequent photographic development.
The preparation of a photographic material generally includes several steps such as precipitation of the crystals in the colloid to form a primitive emulsion, chemical sensitization and spectral sensitization of the emulsion, and coating of the finished emulsion on a support. The photographic properties or overall sensitivity of an emulsion are dependent upon many variables which may be controlled at the various steps in the photographic process. Factors which influence sensitivity of freshly prepared emulsions include the composition (proportion of halides), average size and morphology (shape) of the crystals, the type of chemical and spectral sensitization used, and agents or addenda used to improve coating properties. For example, the most sensitive emulsions usually employ silver bromide crystals. Silver chloride is usually employed in some slow emulsions.
A vexatious problem in the photographic art is the change in photographic properties which occurs upon the aging of emulsion coatings. Photographic characteristics can change during storage as a result of elevated temperature, or as a result of chemical reactions of agents contained in the original coating or of agents from the atmosphere, from the coating support or from the packaging materials. The effects of environment on the aging of emulsions differ with halide composition, chemical sensitization and spectral sensitization.
Some photographic emulsions, in particular, silver chloride emulsions, exhibit an aging pattern in which photographic speed and fog increase during storage. Fog is a deposit of silver or dye that is not directly related to the image-forming exposure, i.e., when a developer acts upon an emulsion layer, some reduced silver is formed in areas that have not been exposed to light. Fog can be defined as a developed density that is not associated with the action of the image-forming exposure, and is usually expressed as "d.sub.min ", the density obtained in the unexposed portions of the emulsion. A density, as normally measured, includes both that produced by fog and that produced by exposure to light.
Several approaches have been described to reduce the storage-related changes in photographic properties. Certain agents can be added to emulsions to attempt to minimize these changes. Agents, known as stabilizers, can be added that decrease the changes in developable fog and/or other sensitometric characteristics of the emulsion coating that occur during storage. Other agents, known as antifoggants or fog restrainers, can be added that decrease the rate of fog density growth during development to a greater degree than they decrease the rate of image growth. Some agents act in both capacities; others may act in only one capacity, or their action may be restricted to particular types of fog development or other aging changes or both. Their quantitative, and sometimes their qualitative, action depends upon the concentration as well as the chemical composition of the agents. Additionally, many agents have limitations in their ability to produce desirable results without producing undesirable side effects. For example, some agents can be added only at specific steps in the photographic process or these agents may, for example, contribute to fog growth or desensitize the emulsion.
Several methods, using certain sulfur-containing (and analogous selenium-containing) compounds, have been described for reducing fogging. For example, U.S. Pat. No. 2,057,764 discloses the incorporation of sulfinic and seleninic acids or salts thereof into the emulsion, the emulsion support or the emulsion coating protective layer, or alternatively, bathing the emulsion layer in solutions of these compounds. U.S. Pat. Nos. 2,394,198 and 2,440,206 disclose the use of certain sulfinic and seleninic acids and their salts in combination with certain thiosulfonate compounds and polythionic acids or salts thereof. European Patent Application Publication 293,917 discloses use of certain thiosulfonate compounds as antifoggants in emulsions in which the silver salts are at least 50 mole% silver chloride. European Patent Application Publication 327,066 discloses the use of certain thiosulfonate compounds in direct positive emulsions.
Similar sulfur-containing compounds have also been described as beneficial in preventing or reducing other types of fog and staining, as well as improving other photographic properties. For example, U.S. Pat. No. 4,198,246 discloses the use of certain thiosulfonate compounds to reduce fog caused by use of thioethers in the precipition step of emulsion preparation, while U.S. Pat. No. 4,276,374 discloses the use of certain thioether compounds to reduce the same type of fog. A combination of certain sulfinates and sulfonate compounds has been disclosed as controlling the formation of stains in developed white background (European Patent Application Publication 305,926). The use of thiosulfonic acid esters in conjunction with 2-equivalent magenta dye couplers has been described to improve the efficiency of color formation (U.S. Pat. No. 4,868,099).
Despite attempts to provide photographic emulsions which maintain photographic speed upon storage, yet control fog growth, the art has not provided a photosensitive material having features that adequately address these considerations.